The use of passive capillary valves as a method for controlling fluid flow in a microfluidic system is discussed in “Hydrophobic microfluidics” by McNeely et al, SPIE Conference on Microfluidic Devices and Systems II, Santa Clara, Calif., September 1999, SPIE Vol. 3877-786X/99.
U.S. Pat. Nos. 5,846,396 and 5,958,344 for example describe a liquid distribution system where liquid is dispensed between a connected feeder channel and a reaction cell via a conduit having a capillary barrier interposed to check the flow. Liquid in the feeder channel remains permanently in contact with the liquid in the capillary channel. As a result, the amount of liquid being distributed cannot be controlled accurately.
U.S. Pat. No. 5,957,149 discloses a fluid dispensing device involving spring-biased mechanical valve members, making its structure complex.
The use of syringes to dispense small amounts of liquids is limited by the difficulty of ejecting into the air because of an insufficient speed, and surface tension effects make it difficult to eject small quantities.
A diagnostics system including a mixing structure arranged on a disc is available under the trademark LabCD™ from Gamera Bioscience. Small quantities of liquids are separated and dispensed by selective rotation of the disc, relying on centrifugal force. This system however has severe limitations in its possible applications.
The ejection of small droplets is also known in the area of ink-jet printing and it has been proposed to apply this technique to generate microarrays of liquid droplets for the production of BioChips, by applying a high acceleration to a print module having nozzles filled with different kinds of liquid from different reservoirs (Displayed at Nanotech 99 Conference, Montreux—Switzerland, November 1999, by Ducreel Zengler).
In these ink-jet systems, up to the instant when a droplet is ejected, the liquid in the nozzles to be dispensed remains in contact with the liquid from the reservoir. This leads to imprecision in the amounts of liquid in the ejected droplets. Ink-jet systems are also subject to drawbacks related to evaporation effects and are unsuitable for ejecting highly viscous liquids. Also, ink-jet ejection heads are liable to contamination and possible degradation of the liquid due to heat generation (therefore unsuitable for applications with biological materials) and are not designed to be thrown away.
WO 99/42805 describes a liquid dispensing device for in-situ metering of reagents in a plate, the reagents being supplied from an external source.
EP-A-0434149 describes a liquid dispensing device in which liquid from a pipette is supplied via elongated capillary ducts under control of a rotary valve.